Hi this is my first post here so hello and I have to say glad I found this forum
I have a 78 Apollo 440-3 engine two gas tanks with aux on thedriver side. I recently figured out the wiring at the dash switch to switch between the two and the 3 wires for the fuel gage. Checked continuity all the way to the tanks and both tanks have it. I picked the aux tank because it's easier to get to.
On the sending unit there's only one wire screwed on that goes to the dash switch color Blue on left and Grey on right. Doesn't it have to have a wire from gound from the body so a total of two wires? That's what I read on all scematics. If anybody has this rv please let me know as I do need a few other things to get it going. It hasn't been driven in over one year and am trying to pass smog but first things first. Thanks in advance for any help

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If the tanks are metal , then the installer relied on the tank it's self for the ground. Over the years, rust on the tanks and mounts may have caused a bad ground condition. If the sending unit required a dedicated ground ( not grounded through the mount flange) it would have 2 terminals , a threaded stud for power and a flat spade, for the ground wire to attach to.78 Dodge P/U, from the factory were that way but they used plastic tanks.
Thermoquad carb, 4bbl or 2bbl Holley with a vacuum governor.
Ca. emissions, ouch!

I am sort of mantaining the RV, even though it has 65k miles it's been rebuil as far as tranny and engine but it sat for over a year in Southern Cali. I need to ask you what you'd suggest as far as the sending unit for the fuel gage. They're both(tanks) metal and the rust issue makes sense. Do I need to remove the tanks or can I play around with the bolts that you mentioned to shake off the rust? Thanks in advance for the help

You can take a new spade terminal and insert it under one of the screws that hold the sending unit to the tank - and run a wire from that to ground. Then there's no more concern about proper grounding.

To do a quick test on the gauge and wires leading to the sending unit, you can remove the single wire you're talking about at the sending unit, then check your guage to see how it's reading. Compare that reading to the reading you get when you ground that wire. One reading should be empty, the other full. Don't remember which is which. -Al

At least I can get to one of them( driver side) the other tank looks enclosed in metal or like it's custom made all square(or cube) not rounded up at the sides like the other. I'll test tomorrow and post. Thanks.

I actually passed smog with the new(smaller) jets I had installed. It's running ok with these. Do I put bigger jets on or keep these? There is a slight difference in street driving but no big loss of power. I didn't have that much power to begin with. Are these RVs supposed to take off pretty slow or do they actually have some power, I never drove any other one so I cannot compare.

The real problems with a jet too small, don't show up until you have to go to wide open throttle for ANY length of time , air to fuel ratio goes out of wack and combustion temps go through the roof , or more correctly through the top of your pistons. Unless your a pro at reading spark plugs, for over temp operation. I'd say put the original, from the factory jet chart, jet size, back in, a little time now could save big $$$ later.
I've seen under size jetting kill an engine at the end of a 11 sec. 1/4 mile run. It can happen that fast.

Generic sending units are a simple rheostat that gives 35 ohms empty (or full) and 240 ohms full (or empty respectively). If there is no second wire to your tank selector then likely as noted above the tank was assumed grounded and that's the second "wire." You can test the ohms (assumes you have an idea how full the tank is) w/an ohm meter- red to sender stud & black to known chassis ground.

IIWMI'd use a ring terminal on one of the sender mount screws w/a black wire to chassis ground.

Thanks Skip426, makes sense, I have to go ask the guy about the original ones. Is there a chart for the Holley 4175 jets? Or should I ask the guy to put the biggest he has? I did notice temperature go up with these jets.
Hey Engineermike also thanks for your post. I found that the aux tank has an opening through the floor that allows access to the tank so I took out the sending unit and opened it. Sure enough the spring like piece inside that slides up and down and determenies the amount of gas was fallen off the stem. My question is can I drill a small hole and solder the piece back on the stem? Will it hold emersed in gasoline? I couldn't locate anything for a 78 Dodge Arrow the closest thing was for a 68-71 Dodge RV unit at $90. I'm pretty good at anything automotive(body and machanical) and electronic(solder etc) so I wouldn't mind the challenge so if anyone knows I'd appreciate. Thanks guys feels good to be on this forum.

The re-builder of the carb should have access to the jet chart, or the original jets from the carb you took to him. You don't want the biggest jets he's got , the black smoke from the tailpipe would, attract the highway patrol & put you back to the emission test in a heartbeat.
If your trying to repair the sending units the wound resistance wire, has to be carefully spaced, or the power will track and give false reading, if memory serves the sending unit works on reduced voltage, 5v comes to mind, supplied by a resistor on the back of the dash, goes through the gauge and the variable resistance to ground of the sending unit.
You can check your repair with an ohm meter.

gica, your electronic solder, would probably be the best, BUT, I don't know where you could get a 100% answer. Inside the fuel tank is the last place you want an arc, or overheated connection. That's why they use reduced volatge, in the fuel gauge circuit.
At the shop, we would not repair an item like this ; liability issues.

Thanks for the answer Skip I thing you misunderstood me I was talking about the sending unit off the tank. I would not solder anything in the tank. I soldered the piece that sends the signal when the floater moves up and down and it worked. But when I tried to adjust it to ge a perfect Empty Full reading on the fuel gage the same bronze looking metal spring like puiece broke in half. So I'm know heading to a pick your parts wrecking yard top try to locate some sending unit off any old 8cyl Dodge van or truck. If not I will have to buy the one on Ebay for $90 but it's for a 69 through 71 model. Do you guys have a place that carries these sending units for Dodge? Thanks

gica, your metal tanks would not be a Dodge item, P/Us and vans had gone plastic by 77, and I'm 99% sure any metal Dodge tank you find will have a lock ring type mount, not the multi bolt mount flange you need.
You can pull a sender from a plastic tank and see if anything can adapt over, but I don't think it will.
Can you contact the e bay seller and ask for a picture , to make sure it's what you need ? Easier than trying to return if you can't use it.
Look all over the tanks you have for a makers tag, maybe the company is still around. If you need to buy new it would be nice to get a 100% correct part.
Can you post a picture of what you have and I'll see if I can give any info on it.